It is conventional to connect a cylinder block, a transmission case and a cast oil pan with each other so as to improve the overall rigidity of the engine power plant and to reduce the vibration of the engine as disclosed in Japanese patent laid open publication (kokai) No. 63-170558.
According to this conventional structure, an annular flange is provided along the upper edge of the oil pan for attachment to the cylinder block, and another annular flange is provided on a surface of the oil pan perpendicular to the axial center line of the crankshaft for attachment to an opposing surface of the transmission case. Mounting bolts are provided for the mounting holes of the flange for attachment to the cylinder block near the intersection between these two annular flanges, and these mounting bolts are a pair of long threaded bolts which pass through the annular flange for attachment to the transmission case. The heads of these long threaded bolts are supported by the lower end of the flange for attachment to the transmission case. Interference is thereby avoided between the mounting bolts and the two annular flanges.
However, according to this conventional arrangement, it is not possible to directly clamp the flange during attachment to the mounting surface of the cylinder block, and it is therefore difficult to achieve a contact pressure sufficient for sealing. As a result, the structure of the power plant is undesirably complex. The long threaded bolts contribute to rigidity of the flange and it is difficult to modify the flange so as to directly clamp it to the mounting surface of the cylinder block.